This invention relates generally to the measurement of molecular orientation in polymeric products and, more particularly, to apparatus for detecting and analyzing variations of fluorescence in a product as it is moved through an incident beam of rotating, linearly polarized light.
It is known in the art that information pertinent to the molecular orientation of polymeric products can be obtained by detecting the intensity of fluorescent radiation emitted from a sample excited by polarized light. Attempts to use available instruments for the on-line inspection of polymeric products such as film, e.g., after the first of two coupled stretching steps, have not been successful for a number of reasons. Where the light source and fluorescence detector are on opposite sides of a film, scanning through a wide sample is difficult and, of course, impossible with opaque films. Where the excitation and emission beams are located on the same side of a sample, they have been disposed angularly and that relationship introduces a bias in measurements of intensity. Although orientation can be determined as a function of birefringence, that measurement is dependent on optical retardation, a function of film thickness. Since thickness must be measured simultaneously with retardation, on-line inspections are not practicable. Instead, it is usually the practice to stop a production line and take a sample in order to obtain information as to the birefringence of a partially processed film. That information must then be related to the finished product by extrapolation.
Another difficulty with available instruments is that the analysis of fluctuating, periodic, sinusoidal signals is complicated by broad-band noise components which originate from sources such as mechanical vibration, photomultiplier tube shot-noise, and stray light from outside the instrument. These effects obscure an accurate measurement of both the degree and direction of molecular orientation in continuously advancing products. In fact, the sinusoidal signals to be measured not only may have amplitude levels below the level of the background noise but also have phase and amplitude characteristics that vary randomly. Such factors have made it practically impossible to relate the axes of fluorescence maxima and minima to the direction of polymer chains.